As the process for producing Bi-based oxide superconductor single crystals, there are known the flux method using a crucible, the floating zone method (hereinafter referred to as the FZ method), and the like. In the flux method, a self flux, a KCl flux, etc. are used as the flux. However, this method involved such problems that (1) the quality of crystals is lowered by contamination from the crucible, leading to a reduction in the superconducting properties and that (2) not only works for separating the flux are required which take plenty of time, but also the crystals to be obtained are so fine that the thickness in the c axis direction is approximately from 0.1 to 0.2 mm.
On the other hand, there is an attempt to increase the thickness in the c axis direction by the FZ method. In this FZ method, two rod-like specimens of polycrystal (feed rods) are vertically supported up and down, the both are made appropriately come near, seed crystals are arbitrarily placed in a gap therebetween, one end of each of the feed rods is heat melted to form a narrow melting zone therebetween, and the resulting feed rods are moved in one direction while being revolved, to grow single crystals.
For example, at this time, by using feed rods which have been subjected to a high-speed melt-passing processing such that the feed rods are dense and that the growth is rendered stable, to have a formulation ratio of metallic elements of Bi/Sr/Ca/Cu of 2.2/1.8/1.0/2.0, the growth is carried out at a rate of 0.5 mm/hr in an atmosphere at an oxygen partial pressure of 22 KPa, to form single crystals having a maximum value of 50.times.5.times.2 mm [e.g. see Teion Kogaku (Low-Temperature Engineering), Vol. 25, No. 5 (1990), pp. 18-24].
Also, by using feed rods prepared so as to have a formulation of metallic elements of Bi.sub.2.2 Sr.sub.x Ca.sub.y Cu.sub.2.0, wherein x and y are each a positive number and x/y of 1.5.about.0.9, the growth is carried out at a rate of 0.5 mm/hr in the atmosphere, to form single crystals having a thickness in the c axis direction of 4 mm (see JP-A-Hei. 3-279287).
However, the single crystals obtained by the above-described FZ method are so inferior with respect to the crystallinity that different phases or fine pores, or cracks, etc. are found in the particles. Accordingly, these single crystals are not suitable as large-size single crystals having a good quality to be required as single crystals for the measurement of generally precise physical properties or of an anisotropic effect, etc.